Internal combustion engines that are at times or predominantly operated with a lean air-fuel mixture produce nitrogen oxides NOx (mainly NO2 and NO), thus requiring NOx-reducing measures. An engine-related measure aimed at reducing NOx raw emissions in the exhaust gas is the exhaust-gas return approach, in which part of the exhaust gas of the internal combustion engine is returned to the combustion air, as a result of which the combustion temperature is lowered, thereby reducing NOx formation. However, the exhaust-gas return approach is not always sufficient to meet statutory NOx limit values, which is why an active exhaust-gas after-treatment is additionally needed in order to lower the final NOx emissions. A known NOx exhaust-gas after-treatment calls for the use of NOx storage catalysts that, during lean operation (at λ>1), store nitrogen oxides in the form of nitrates and, at short intervals with a rich exhaust-gas atmosphere (λ<1), they desorb the stored nitrogen oxides and reduce the reductants that are present in the rich exhaust gas so as to form nitrogen N2.
Another known approach for converting nitrogen oxides that are present in the exhaust gases of internal combustion engines that can run with lean mixtures is the use of catalytic converter systems that function according to the principle of selective catalytic reduction (SCR). These systems comprise at least one SCR catalytic converter that, in the presence of a reductant that has been added to the exhaust gas, usually ammonia NH3, converts the nitrogen oxides that are present in the exhaust gas into nitrogen and water. Here, the ammonia can be metered into the exhaust-gas stream from an aqueous ammonia solution or it can be obtained from a precursor compound, for example, urea in the form of an aqueous solution or solid pellets by means of thermolysis and hydrolysis. A new approach for ammonia storage in vehicles comprises NH3 storage materials that reversibly bind ammonia as a function of the temperature. In particular, metal ammine storage complexes are known in this context such as, for example, MgCl2, CaCl2 and SrCl2, which store ammonia in the form of a complex compound so as to be present, for example, as MgCl2(NH3)x, CaCl2(NH3) or SrCl2(NH3). The ammonia can be released from these compounds once again through the input of heat.
Moreover, in order to achieve a further reduction of the nitrogen oxide emissions, two-stage SCR catalytic converter systems are known that comprise a first SCR exhaust-gas treatment unit arranged near the engine as well as an SCR catalytic converter that is arranged downstream from it and that is normally located on the undercarriage of the vehicle. In this context, the SCR exhaust-gas treatment unit arranged near the engine can especially be configured as a particulate filter that has an SCR catalytic coating. Consequently, such a catalytically coated particulate filter (also called SDPF) combines the functions of the retention of soot particles as well as of the selective catalytic reduction of nitrogen oxides in the presence of a reductant, especially NH3. Since the SCR exhaust-gas treatment unit or the SDPF is arranged near the engine, this SCR component is quickly heated up to its operating temperature. This permits an early triggering of the reductant metering process and thus an improved NOx conversion during the entire driving cycle. The downstream SCR catalytic converter serves to further improve the NOx conversion as well as to prevent the emission of a reductant slip of the SCR unit that is arranged near the engine.
However, the SCR exhaust-gas treatment unit arranged near the engine, especially in the form of an SDPF arranged near the engine, only permits the use of a limited quantity of catalytic SCR coating since otherwise, unacceptable exhaust-gas counter-pressures would arise. Consequently, the NOx conversion rate of the SCR exhaust-gas treatment unit arranged near the engine is often not sufficient to meet low NOx emission requirements over the entire operating family of characteristics. For this reason, the downstream SCR catalytic converter continues to be necessary, whereby its volume can be reduced as compared to SCR concepts that use exclusively an undercarriage SCR catalytic converter.
German patent application DE 102 47 989 A1 describes an SCR catalytic converter system that has an oxidation catalyst as well as a downstream SCR catalytic converter. Here, the feed means for feeding the reductant into the exhaust-gas stream is integrated into the oxidation catalyst. This is meant to achieve a good distribution of the reductant as well as to shorten the mixing segment.
German patent application DE 10 2005 055 240 A1 discloses an SCR catalytic converter system with an undercarriage SCR catalytic converter. Upstream from the undercarriage SCR catalytic converter, the exhaust-gas system is divided into a main exhaust-gas system and a secondary exhaust-gas system. A particulate filter is installed in the main exhaust-gas system. Upstream from the particulate filter, there is an oxidation catalyst with an upstream hydrocarbon injection means. The exothermal catalytic oxidation of the fed-in hydrocarbons in the oxidation catalyst produces the necessary temperature for the regeneration of the downstream particulate filter. In the secondary exhaust-gas system, there is likewise an oxidation catalyst as well as a continuously regenerating particle catalyst that has an oxidation-catalytic coating for the particle oxidation. During normal operation, the entire exhaust-gas stream is conveyed through the main exhaust-gas system, where the particles are held back in the particulate filter. During the regeneration operation of the particulate filter, a partial exhaust-gas stream is conveyed through the secondary exhaust-gas system and the other partial stream is conveyed through the particulate filter that is to be regenerated in the main exhaust-gas system. Due to the reduced exhaust-gas mass flow in the main exhaust-gas system, the amount of energy needed to raise the temperature is reduced, as a result of which energy is supposed to be saved and moreover, the SCR catalytic converter is supposed to be less stressed thermally.